To get back on topic, I just looked at the manual for my new 528xi and saw that, to mo surprise, BMW now recommends 91 octane gas but says that 89 octane gas can be used without a problem. It does state that 89 octane gas may cause some knocking in hotter climates when first starting the engine but will not cause any short or long-term damage to the engine and is fully endorsed as a fuel choice by BMW. Go figure. I also looked on the gas cap, and it states "89 octane minimum." Big change from my E46!